John Henry Dekker

John Henry Dekker, known to all as Jack, passed away peacefully in his home and surrounded by family on May 17, 2026, in Appleton, Wisconsin, at the age of 79. He leaves behind a legacy defined by an intense curiosity, a fiercely independent mind, and an enduring passion for science and the natural world.

A Life of Inquiry

Born in Oak Park, Illinois, to Tunis Dekker and Marguerite Spelman, Jack was raised in Oak Park and East Lansing, Michigan. As a young man, he served in the Army I Corps during the Vietnam War- an experience that became central to his personal growth and shaped his resilient, philosophical outlook on life.

Jack went on to earn a PhD in plant physiology, embarking on a distinguished career as a professor and researcher in weed biology at Iowa State University. A true pioneer and nonconformist in his field, Jack authored numerous peer-reviewed publications. He devoted his career to a specific weed that he loved and traveled the world to study: the foxtail (Setaria spp.). His passion for botany never waned; even in retirement, he self-published his definitive book in 2016, Evolutionary Ecology of Weeds, which remains available on Amazon.

Passions, Tuscany, and "Leave It To Beaver-Land"

Jack was a nonconformist by nature and man of deep, meticulous interests. His world travels took him all across North America, Europe, and Asia, including a research trip along the entire Trans-Siberian Railway. He was a dedicated walker, logging up to three miles a day until just a few weeks before his passing. A voracious reader, his library was filled with books spanning history, science, mystery novels, Patrick O'Brian, Camilleri, Murakami, and Balzac. His taste in music and CD collection was legendary, encompassing many genres from classical to grunge. An analytical thinker and an observer at his core, Jack was a dedicated writer who journaled throughout his entire adult life, documenting his experiences and the world around him. Jack was also a profound oenophile; he curated an extensive wine cellar on two continents and kept characteristically meticulous records of every vintage he enjoyed. Following in the footsteps of his mother, grandfather, and uncle, Jack created art throughout his life through drawing, photography, and painting. A master of organization and devotee of LocknLock containers, Jack left his life archived in good order.

For the last twenty years of his life, Jack split his time between the United States and Tuscany, Italy. Tuscany was where he shared a beautiful life with his beloved partner, Francesca, and her wonderful family and friends who meant the world to him. At his home in Appleton, Jack lovingly maintained an extensive collection of Francesca's distinctive paintings and artwork. When Francesca passed away last November, it utterly broke his heart, and his family takes comfort imagining them reunited.

Three years ago, Jack moved to Appleton, Wisconsin, to be close to his family. A dedicated homebody who loved tackling projects around the house, he quickly fell in love with his house, neighborhood, neighbors, the Fox River, and regular trips to Manderfield's Bakery. He fondly nicknamed Appleton "Leave It To Beaver-Land," deeply appreciating its peacefulness and traditional community values.

Family and Legacy

Jack’s intelligence, passion, and authenticity live on through his family. He is survived by his children, Beatrix Dekker and Robin Ahrens; his cherished grandchildren, Ivy, Mina, and Ronan; and his former wife and the mother of his children, Corrin Seaman. He is also survived by his siblings, Paul Dekker, Deborah Komar, and Marguerite Dekker, as well as Francesca’s childrenMartina, Francesco, and Giulia- who were a deeply special part of his life.

He was preceded in death by his parents, Tunis and Marguerite, and his beloved partner, Francesca.

Jack’s most deeply held values were curiosity and autonomy. In the final months of his life, his intellectual world found a vibrant new focus when his early interest in radios was reignited. True to his brilliant, unconventional nature, he began drafting a scholarly paper examining the complex similarities and connections between radio circuitry and plant biology. Ever the visionary, in his last days he also began weaving profound connections between neurodiversity and these intricate systems.

Memorial and Contributions

A celebration of Jack's life will be held at a future date, and details will be shared as they are finalized. In the meantime, friends and family are invited to share memories, stories, and photos on his dedicated memorial page on Ever Loved, and receive celebration of life updates, at https://everloved.com/life-of/john-dekker/ 

In lieu of flowers, the family requests that donations be made in Jack’s honor to the Autistic Self Advocacy Network (ASAN) via the link provided on his memorial page.